The invention relates to apparatuses for determining layer thicknesses on a tape and/or for determining a degree of contamination of a surface of a tape, the surface being contaminated by particles.
The tape or the tape-shaped body can have a plurality of layers and can be used e.g. in semiconductor fabrication for producing components appertaining to organic electronics, e.g. organic LEDs, or solar cells. In these applications, the tape is typically designed to be very thin or film-like, that is to say that the tape has a thickness which is typically in the micrometers range.
Components appertaining to organic electronics which can be fabricated from a tape-shaped substrate, for example, are sensitive to water and oxygen, inter alia, for which reason it is necessary to encapsulate these components relative to the environment. Such an encapsulation could be effected by glass or with the aid of metal hoods. However, such an encapsulation generally cannot be used for flexible, film-like and/or printed electronic components on account of its stiffness.
In order to encapsulate flexible components appertaining to organic electronics which are produced from tape-shaped substrates with a plurality of layers, it is possible to use, on both sides of the components, thin barrier layers (e.g. multilayers composed of Al2O3 and/or ZrO2) having a sufficiently low water vapor transmission rate (WVTR), cf. the article “Al2O3/ZrO2 Nanolaminates as Ultra-high Gas Diffusion Barriers—a Strategy for Reliable Encapsulation of Organic Electronics”, by J. Meyer et al. Adv. Mater. 18, 1845 (2009) or the article “Thin Film Encapsulation of Top-Emitting OLEDs using Atomic Layer Deposition” by Thomas Riedl et al., in Solid-State and Organic Lighting, OSA Technical Digest (CD) (Optical Society of America, 2010), paper SOWB5.
It is problematic for the production of organic electronic components, however, if there are particles present on the surface to which the barrier layer is intended to be applied, since the particles firstly can form microchannels which enable water and/or oxygen from the environment to pass through the barrier layer, and secondly can lead to electrical short circuits between the electrodes of the organic components. Water and oxygen are critical in particular in the production of organic LEDs (OLEDs), since they can lead to oxidation of the cathode, which in OLEDs generally consist of a base metal (e.g. aluminum), with the result that current can no longer flow locally (cf. “White Paper on the Characterization of Thin-film Barrier Layer for Protection of Organic Light-Emitting Diodes” by Peter van de Weijer, Ton von Mol, 10 Sep. 2009, retrievable on the Internet at “www.fast2light.org”). This can lead to black points on the surface emitters and possibly to failure of the OLED.
Organic electronic components are generally processed either in clean rooms (e.g. having permissible particle sizes of a maximum of 0.5 μm), in a vacuum or under a protective gas atmosphere. In this case, care should be taken to ensure that the substrates or surfaces used are free of particles. In particular, no particles which penetrate the barrier layers and the electrodes, in particular the cathode (in the case of an OLED), should be present in the region of an electronic component. Smaller particles which are still covered by the barrier layer or which are present only in the inner layers of the component are less critical, by contrast. The minimum critical particle size is therefore the sum of the smallest occurring layer thicknesses of barrier layer and electrode, i.e. typically in the region of approximately 0.15 μm.
During the process for producing OLEDs, the largest particles are produced when the anode (generally composed of indium tin oxide (ITO)) is applied to the carrier material by a sputtering process, wherein the particle sizes in this process step can be approximately 5 μm, cf. “Impact of Particulate Contaminants on the Current Leakage Defect in OLED Devices” by Masaru Nagai, J. Electrochemical Soc. 154, J387 (2007). However, these particles can readily be removed by cleaning Residual ITO particles may be critical with regard to short circuits, but generally not for the diffusion of water on the anode side, since there the barrier layer is applied before the ITO layer and is therefore not penetrated by particles in that they do not arise until as a result of the ITO sputtering. Smaller particles that do not penetrate the conductive layers (ITO anode, shunting lines, conductive polymer layers) are comparatively noncritical, by contrast.
It is known to observe the surfaces of substrates with the aid of cameras. However, the resolution (>10 μm) of the cameras is generally not sufficient for identifying or detecting small particle sizes of less than 10 μm. Other solutions having a high resolution use a point-type measurement (cf. “Light Scattering by Sub-half Micron Spherical Particles on Silicon and Oxidized Silicon Surfaces”, Bawolek et al. Proc., ICCCS 485 (1992)) and are therefore typically too slow to examine the entire surface (in a scanning manner) in situ during a production process or are generally unsuitable for process monitoring (e.g. solutions with electron microscopes).
Besides determining the degree of contamination of the surface of a tape-shaped body by particles or determining the particle density on the surface, for the processing of the tape it is also advantageous to determine the thickness or the refractive index of the tape or of individual layers of the tape with a high spatial resolution. Typically, the coarse layer construction of the tape is already known in this case. The measurement accuracy when determining the layer thickness (in the case of average thicknesses of the order of magnitude of approximately 100 nm) should be as high as possible (typically <2 nm), however, and the measurement should be able to be carried out at high speed and with a spatial resolution (grid) of an order of magnitude of approximately 100 μm, which cannot be accomplished by conventional apparatuses for thickness measurement at the present time.